`PbList` checks element type twice on each add

I don't think this can be fixed, but it's still worth documenting this issue somewhere probably..

`PbList.add` currently checks the element type, and then calls `GrowableList.add`, which checks the element type again.

Here's the relevant Wasm when compiled with `-O2`: (the default optimization setting when building Flutter apps)

```wat
(func $PbList.add (;353;) (param $var0 (ref $Object)) (param $var1 (ref null $#Top)) (result (ref null $#Top))
(local $var2 (ref $PbList))
(local $var3 (ref $_Type))
(local $var4 (ref $#Closure-0-1))
local.get $var0
ref.cast $PbList
local.tee $var2
struct.get $PbList $field2
local.set $var3
local.get $var1
ref.is_null
if (result i32)
local.get $var3
struct.get $_Type $field2
else
local.get $var3
local.get $var1
ref.as_non_null
call $_InterfaceType._checkInstance
end
...
)

(func $GrowableList.add (;174;) (param $var0 (ref $Object)) (param $var1 (ref null $#Top)) (result (ref null $#Top))
(local $var2 (ref $_Type))
(local $var3 (ref $WasmListBase))
(local $var4 (ref $#Top))
(local $var5 i64)
(local $var6 i32)
local.get $var0
ref.cast $WasmListBase
local.tee $var3
struct.get $WasmListBase $field2
local.set $var2
local.get $var1
ref.is_null
if (result i32)
local.get $var2
struct.get $_Type $field2
else
block $label0 (result i32)
i32.const 1
local.get $var2
struct.get $_Type $field0
local.tee $var6
i32.const 8
i32.eq
br_if $label0
drop
local.get $var1
ref.as_non_null
local.set $var4
local.get $var6
i32.const 12
i32.eq
if
local.get $var2
local.get $var4
call $_InterfaceType._checkInstance
br $label0
end
local.get $var2
local.get $var4
local.get $var2
struct.get $_Type $field0
i32.const 287
i32.add
call_indirect (param (ref $_Type) (ref $#Top)) (result i32)
end $label0
end
i32.eqz
if
local.get $var1
local.get $var2
call $_TypeError._throwAsCheckError
end
...
)
```

Now consider a packed field decoding loop, with #959.

For each 4-byte integer, we box the integer, and then check the type of it twice.

To have an idea of how much we could save, if I omit type checks in the generated code with `-O4`, here's before and after:

```
// Before
protobuf_PackedInt32Decoding(RunTimeRaw): 49775.0 us.

// After
protobuf_PackedInt32Decoding(RunTimeRaw): 36812.5 us.
```

That's 26% of decoding time wasted on redundant type checks.

Start by reviewing the PbList.add and GrowableList.add entry points described in the issue, then compare the packed field decoding loop referenced with #959. Document that element types are checked twice and include the reported protobuf_PackedInt32Decoding benchmark; the issue does not name a documentation file or test to update.

Written by the indexing model from the issue text.